With the growing use of portable electric products such as video cameras, mobile phones, and portable computers, significance of secondary batteries being mainly used as a power source on which the portable electric products work are increasing.
Generally, as opposed to a disposable primary battery, a secondary battery is rechargeable and is being studied very actively with the development in high-tech fields, for example, digital cameras, cellular phones, laptop computer, power tools, electric bikes, electric vehicles, hybrid electric vehicles, high-capacity energy storage systems, and the like.
In particular, a lithium secondary battery has a high energy density per unit weight and allows quick charging, when compared to other conventional secondary batteries such as a lead storage battery, a nickel-cadmium battery, a nickel-hydrogen battery and a nickel-zinc battery, and thus, its use is on an upward trend.
A lithium secondary battery has an operating voltage higher than or equal to 3.6V, and is used as a power source of portable electronic appliances or high power devices such as electric vehicles, hybrid electric vehicles, power tools, electric bikes, energy storage systems, and uninterruptible power supplies (UPS) by connecting a plurality of batteries in series or in parallel.
A lithium secondary battery has three times higher operating voltage than that of a nickel-cadmium battery or a nickel-metal hydride battery and an excellent characteristic of energy density per unit weight, and thus, is being increasingly used.
A lithium secondary battery may be classified into a lithium ion battery using a liquid electrolyte and a lithium ion polymer battery using a solid polymer electrolyte, based on the type of an electrolyte. Also, a lithium ion polymer battery may be divided into an all-solid-state lithium ion polymer battery containing no electrolyte liquid and a lithium ion polymer battery using a gel polymer electrolyte containing an electrolyte liquid, based on the type of a solid polymer electrolyte.
A lithium ion battery using a liquid electrolyte is generally sealed hermetically by welding using a cylindrical or prismatic metal can as a container. A can-type secondary battery using a metal can as a container has a fixed shape, which has limitations on design of electric products using it as a power source as well as on volume reduction. Accordingly, a pouch-type secondary battery fabricated by putting an electrode assembly and an electrolyte into a pouch casing made from films and forming a seal has been developed and is being used.
However, a lithium secondary battery has a risk of explosion when overheated, so ensuring safety is one of the important challenges. Overheat occurs in a lithium secondary battery by various reasons, and one of them is a flow of overcurrent beyond the limit through a lithium secondary battery. When an overcurrent flows, a lithium secondary battery generates heat by Joule heating and the temperature inside the battery increases rapidly. Also, a rapid temperature rise brings about a decomposition reaction of an electrolyte solution, causing a thermal runaway phenomenon, and in the end, results in explosion of the battery. An overcurrent occurs due to dielectric breakdown between a cathode and an anode caused by penetration of a pointed metal object through a lithium secondary battery or shrinkage of a separator interposed between the cathode and the anode, or when a rush current is applied to the battery due to an abnormal condition of an external charging circuit or load being connected.
Accordingly, to protect a lithium secondary battery from an abnormal situation such as occurrence of an overcurrent, the battery is used in combination with a protection circuit, and generally the protection circuit includes a fuse device to irreversibly disconnect a line through which a charging or discharging current flows in the event of an overcurrent.
FIG. 1 is a circuit diagram illustrating a layout and an operating mechanism of a fuse device in the construction of a protection circuit connected with a battery pack including a lithium secondary battery.
As shown in the drawing, the protection circuit includes a fuse device 1 to protect the battery pack when an overcurrent occurs, a sense resistor 2 to sense an overcurrent, a microcontroller 3 to monitor the occurrence of an overcurrent and operate the fuse device 1 when an overcurrent occurs, and a switch 4 to perform a switching operation to cause an operating current to flow into the fuse device 1.
The fuse device 1 is installed on a main line connected to an outermost terminal of the battery pack. The main line refers to a wire through which a charging or discharging current flows. In the drawing, the fuse device 1 is illustrated as being installed on a high potential line (Pack+).
The fuse device 1 is a 3-terminal element; two terminals are connected to the main line through which a charging or discharging current flows and the rest is connected to the switch 4. Also, on the inside, the fuse device 1 includes a fuse 1a which is directly connected to the main line and melts at a particular temperature, and a resistor 1b which applies heat to the fuse 1a. 
The microcontroller 3 monitors whether an overcurrent is occurring or not by periodically detecting the voltage across both ends of the sense resistor 2, and when an occurrence of an overcurrent is detected, turns on the switch 4. Then, the electric current flowing through the main line is bypassed to flow toward the fuse device 1 and applied to the resistor 1b. Thus, Joule heat generated from the resistor 1b is transmitted to the fuse 1a and increases the temperature of the fuse 1a, and when the temperature of the fuse 1a reaches a melting temperature, the fuse 1a melts and as a consequence, the main line is irreversibly disconnected. When the main line is broken, the overcurrent does not flow any longer and the problem caused by the overcurrent may be solved.
However, the related art described in the foregoing has many problems. That is, when a failure occurs in the microcontroller 3, the switch 4 does not turn on even in the situation where an overcurrent occurs. In this case, an electric current does not flow into the resistor 1b of the fuse device 1, and the fuse device 1 does not work. Also, a separate space for disposing the fuse device 1 within the protection circuit is needed, and a program algorithm for controlling the operation of the fuse device 1 needs to be loaded in the microcontroller 3. Therefore, there are drawbacks of reduced spatial efficiency of the protection circuit and increased load of the microcontroller 3.